vii LITTORAL AND PELAGIC REGIONS OF LAKES 20/ 



agitation of its waters, the affluence of streams and drainage, 

 and the constant changes of temperature. The water in this 

 region generally contains a good deal of solid matter in sus- 

 pension, while the shelving hanks of the lake support a wealth 

 of vegetable growth, both of Algae and of Phanerogams, down 

 to about 20-25 metres. At this depth the dayhght does not 

 penetrate sufficiently to admit of the growth of green plants, so 

 that this region marks the limit, both pliysical and biological, 

 between the littoral and the abyssal zones. In this littoral 

 region there flourish a great quantity of Entomostraca, most of 

 which are also found in small ponds where similar conditions 

 of life prevail — the pelagic species only penetrating rarely, and 

 by accident, into its waters. At the beginning of July Mi: H. 

 O. S. Gibson and myself found that the weedy littoral region of 

 Grasmere contained almost entirely large quantities of the 

 Cladoceran Eurycercus lamellatus, and a number of Cycloids 

 fuscus and C. strenuus. In the littoral zone of large lakes, 

 Amphipods, Isopods, and fresh-water shrimps may also be met 

 with, but this applies more to the lakes of the Tropics and of 

 the Southern Hemisphere. 



The pelagic ^ region is distinguished from the littoral by the 

 greater purity and transparency of its waters, and by the 

 relative stability of the temperature, the annual range, even at 

 the surface, in Geneva being from 4°— 20° C, while at 100 metres 

 the water has a uniform temperature of 4° or 5° C. The upper 

 strata are, of course, brightly illuminated, but at 20 metres 

 there is hardly sufflcient light for green plants to grow, and at 

 100 metres it is completely dark. The inhabitants of this 

 region, known collectively as plankton, spend their whole life 

 swimming freely in the water, sometimes at the surface and 

 sometimes in the deeper strata. They consist chiefly of 

 Diatoms, Protozoa, Eotifera, and Crustacea. The pelagic 

 Crustacea, especially the Cladocera, are often the most curiously 

 and delicately built creatures. Leptodora hyalina, which is 

 quite transparent, is the largest of them, attaining to three- 

 quarters of an inch in length, though Bythotreiolies longi- 

 inanus is nearly as large if we include the immense spine 

 which terminates the body. ]Iolo2}ediu7n gihherum, which is 



' Consult Apstein, "Das Siisswasserplankton,'' Kiel and Leipzig, 1896 ; and 

 Arch. f. Hydrobioloyie u. PlanMonlcundc, numerous papers. 



